1. Field of the Invention
The present invention relates generally to cargo transfer cranes and particularly to a mobile hoisting crane for loading and unloading cargo ships.
2. Description of the Prior Art
Used upon a dock at a harbor installation, a cargo gantry crane provides a vital link in the movement of goods between land based transportation systems and sea-going vessels by accomplishing the Herculean task of loading and unloading cargo therebetween. Typically, such gantry cranes move along rails that run lengthwise alongside a ship by means of a self-propelled wheel-type motor bed. A vertical gantry tower rises above the motor bed to support a horizontal load-bearing cantilevered boom, such that the free end of the boom may hang over a ship's hold. Cargo is hoisted by means of a spreader suspended from a trolley coupled to the boom. The trolley moves the hoisted cargo between the ship and the dock lengthwise along the underside of the boom. The crane is operated from a cab suspended high overhead attached to the trolley. When not in use, the gantry boom includes a pair of pivot pins positioned in front of the tower that enable the free end of the boom to rotate on a horizontal axis in a drawbridge like fashion to a vertical position thereby creating an unobstructed berth for the ships while docking.
Typically, one or two gantry cranes handle the task of loading or unloading cargo, whereby such cranes must access several hatches to the ship's hold. Such a scheme requires the gantry crane to selectively move lengthwise alongside the ship in order to position the boom over each of the hatches. During such maneuvers, the gantry crane operator ensures the boom remains unobstructed by relatively stationary vertically-extending objects, such as the ship's masts and radar antennas. Additionally, longshoremen and trucks may obstruct the ground path of the gantry crane, so it is also very important for the operator to direct his or her attention to the rails to avoid injury to people or other objects that may be in the path of the crane as it gantries along the rails. Focus of the operator's attention on the ground path may divert his attention from the gantry boom as it translates over the ship.
A collision of the gantry boom during movement of the gantry crane can damage the crane or, more frequently, inflict severe damage to the ship's mast or radar antenna. This can affect the sea worthiness of the ship thus dictating that the ship remain in port until repairs are completed. In out of the way ports, where repair services are not readily obtainable with any degree of dispatch, this problem can work a severe economic hardship on the ship's operation. The time lost by the ship laying in port may even have a greater economic impact than the cost of repairs on the ship's operation. Thus, it has long been established that there exists a need for a reliable system for detecting the approach of the crane on obstacles in the gantry boom's path so that progress of the crane can be interrupted pending clearance of the path.
In recognition of this need, devices have been proposed to detect objects located in the path of a cargo gantry crane. One such system, sold by Bewa, Inc., Sharon, Pa., for bewa-GmbH of Germany, under the model designation Colwar III, employs an array of ultrasonic receivers arranged lengthwise along a gantry crane boom in spaced apart relation to detect the boom moving near an object. Because such receivers have a limited detection range, the length of the boom dictates the number of receivers required for adequate coverage. These receivers tend to be costly and for larger cranes may be cost prohibitive because of the extensive number of receivers required along the boom. Additionally, the receivers may be subject to error due to interference caused by the trolley moving along the boom.
Similarly, devices have been proposed to determine the position of a spreader in relation to objects underlying the spreader for the lowering of cargo thereon. In Tax et. al., U.S. Pat. No. 5,048,703, a container crane installation is disclosed having a remote recognition system and a position recognition system fitted on a spreader suspended by rigging from a crane trolley. The remote recognition system ascertains the distance of the spreader from the objects in the spreader's path, such as a ship's hatch, hold or cargo containers, by calculating the round-trip time of a pair of laser pulses projected from the spreader to the object and back. Intended to ascertain the position of the spreader with respect to the trolley, the position recognition system determines the distance and pitch of the spreader in relation to the trolley by calculating the round-trip time and lateral displacement of a pair of reflection beams projected from the spreader to the trolley and back. In each of the recognition systems disclosed, the respective beams are directed along the vertical path of a spreader at objects having large planar surfaces that provide ideal identifiable reflective surfaces. Such systems do not overcome the problem of detecting a vertically extended object in the path of a gantry boom, such as a ship's mast with a small cylindrical surface, where such an object provides a poor or non-reflective surface.
Thus, the need exists for a cost effective reliable system to detect objects that may appear in the path of the gantry crane as it gantries lengthwise alongside a ship. Such a system must be capable of detecting objects, error free, even under extreme weather conditions, in dirty and dusty environments and while other lighting devices are operating near the gantry crane.